My quantum algorithm won’t break the internet… yet

MIT’s Peter Shor explains why he devised an algorithm for a quantum computer that could unravel our online data encryption

Internet security relies on the fact that our computers can’t break its cryptosystems. But the quantum algorithm you devised has the potential to do just that. Why create it?My motivation was to see what you can do with a quantum computer. An earlier quantum algorithm worked by using periodicity – the tendency of some number sequences to regularly repeat. This is related to factoring, or finding which smaller numbers big numbers are divisible by, so I thought a quantum computer might be able to factor large numbers. As internet cryptosystems rely on the fact that current computers cannot factorise big numbers, I figured a powerful enough quantum computer could break these systems.

Quantum cryptography can’t be broken by factorisation. Could it one day replace standard cryptosystems?For short distances it wouldn’t be too hard to build a quantum key distribution network to encrypt data. Over longer distances, you would need quantum repeaters every 50 kilometres or so on the fibre-optic network, as it’s difficult to maintain a quantum state over long distances. Even if they are cheap by then, it’s a lot of investment.

How much harder is it to write an algorithm for a quantum computer?Much harder. Quantum computers rely on a form of interference – basically the same phenomenon as light wave interference, in a more mathematical setting. Computational paths to the right answer need to interfere constructively, while those to the wrong answer should interfere destructively. We don’t yet know how to do that in general.

Why write quantum algorithms when we don’t yet have proper hardware to run them?The more things that you can do with quantum computers, the more important it is to build them. For example, you should be able to use them to better design drugs using quantum effects that predict the molecules’ chemistry. Right now, drug companies use conventional software to simulate those effects, but you might do better if you could use an actual quantum computer.

Do you think consumer quantum computers will ever be commonplace?I’m not sure they will ever take off. You are not going to have a quantum computer on your desk – they have to be kept at 0.15 Kelvin (-273 °C) for a start. But there probably will be ones physicists can access through the internet.

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Peter Shor is a computer scientist at the Massachusetts Institute of Technology. His quantum algorithm could crack the encryptions that protect our data online – but only if a powerful enough quantum computer is built